Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue Control of cerium oxidation state through metal complex secondary structures† Cite this: Chem. Sci.,2015,6, 6925 Jessica R. Levin, Walter L. Dorfner, Patrick J. Carroll and Eric J. Schelter* A series of alkali metal cerium diphenylhydrazido complexes, Mx(py)y[Ce(PhNNPh)4], M ¼ Li, Na, and K, x ¼ 4 (Li and Na) or 5 (K), and y ¼ 4 (Li), 8 (Na), or 7 (K), were synthesized to probe how a secondary coordination sphere would modulate electronic structures at a cerium cation. The resulting electronic structures of the heterobimetallic cerium diphenylhydrazido complexes were found to be strongly dependent on the identity + + of the alkali metal cations. When M ¼ Li or Na , the cerium(III) starting material was oxidized with concomitant reduction of 1,2-diphenylhydrazine to aniline. Reduction of 1,2-diphenylhydrazine was not + observed when M ¼ K , and the complex remained in the cerium(III) oxidation state. Oxidation of the Received 18th July 2015 cerium(III) diphenylhydrazido complex to the Ce(IV) diphenylhydrazido one was achieved through a Accepted 10th August 2015 simple cation exchange reaction of the alkali metals. UV-Vis spectroscopy, FTIR spectroscopy, DOI: 10.1039/c5sc02607e electrochemistry, magnetic susceptibility, and DFT studies were used to probe the oxidation state and Creative Commons Attribution 3.0 Unported Licence. www.rsc.org/chemicalscience the electronic changes that occurred at the metal centre. Introduction chemistry we recently have focused on understanding the ther- modynamic and kinetic factors that underlie cerium redox Cerium is unique among the lanthanides because of its acces- reactions. We demonstrated that in the cerium heterobimetallic IV/III + 1–3 ¼ sible +4 oxidation state (E (Ce ) ¼ 1.40 V vs. Fc/Fc ). frameworks, [M3(THF)n][Ce(BINOLate)3]M Li, Na, K, Cs and 0 Considering its standard reduction potential, Ce(IV) complexes BINOL ¼ (S)-1,1 -bi-2-naphthol, the secondary coordination are best known as one-electron oxidants in inorganic and sphere, namely the identity of M+, impacted the rates and This article is licensed under a organic syntheses, as well as in materials chemistry.4–16 For product outcomes of electron transfer (ET) reactions.36,37 Given example, ceric ammonium nitrate (CAN), has been used in these observations, we were compelled to investigate cerium water oxidation, oxidation of alcohols, oxidative carbon–carbon heterobimetallic complexes with redox active ligands to express Open Access Article. Published on 11 August 2015. Downloaded 9/27/2021 7:22:12 AM. coupling reactions, and in oxidative deprotection of ketones and modulate cerium–ligand intramolecular redox chemistry. and acetals.4,10 In materials chemistry cerium has been used in Herein, we report that the choice of alkali metal cation in the ¼ ¼ both oxidative and reductive contexts. Cerium(IV) dioxide (ceria) complexes Mx(py)y[Ce(PhNNPh)4], M Li, Na, and K, x 4 and related materials are applied in catalytic redox cycling (Li and Na) or 5 (K), and y ¼ 4 (Li), 8 (Na), or 7 (K), resulted in devices, such as in fuel cells,15,17–21 active supports in 3-way variable electronic structures. Our results showed the smaller, catalytic converters,22–24 for the water gas shi reaction,25–28 and harder alkali metal cations Li+ and Na+ stabilized the tetravalent + in heterogeneous catalysis for organic reactions and fuel cerium cation whereas the soer K formed a cerium(III) production.11,12,29–32 complex. To the best of our knowledge, these results are the rst We have studied the electrochemical behaviour of a variety of examples of the use of secondary coordination sphere effects to cerium complexes,33,34 and found that, despite the isolated nature modulate the oxidation state of a lanthanide cation. of the cerium 4f1 electron, electron donating ligands shi the IV/III Ce(IV/III) redox potential to reducing values, e.g. E1/2(Ce [2- t ¼ + t ( BuNO)py]4) 1.95 V vs. Fc/Fc , where 2-( BuNO)py is N-tert- butyl-N-2-pyridylnitroxide.33,35 To further expand cerium redox Results and discussion Synthesis and structural characterization of 1–3 + Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of Zdilla and coworkers recently reported a Li heterobimetallic Pennsylvania, 231 South 34th St., Philadelphia, Pennsylvania 19104, USA. E-mail: diphenylhydrazido complex that effectively stabilized high val- [email protected]; Tel: +1 215-898-8633 ent Mn(IV) cations despite the reducing character of the ligand † Electronic supplementary information (ESI) available: NMR spectra, UV-Vis – and the oxidizing character of Mn(IV) (see Scheme 1).38 40 spectra, FTIR spectra, Evans' method data, eld dependence data, XAS spectra, Intrigued by these results and the relative scarcity of electro- electrochemical data, DFT coordinates and rendered molecular orbitals. CCDC 1404761–1404763. For ESI and crystallographic data in CIF or other electronic chemical properties reported for anionic nitrogen donors at format see DOI: 10.1039/c5sc02607e cerium,41 we hypothesized that 1,2-diphenyl hydrazido ligands This journal is © The Royal Society of Chemistry 2015 Chem. Sci.,2015,6, 6925–6934 | 6925 View Article Online Chemical Science Edge Article Because of the poor solubility of K5(py)7[Ce(PhNNPh)4](3)in – Et2O pyridine mixtures, 3 was prepared in neat pyridine by reaction of Ce[N(SiMe3)2]3 with 4 equiv. 1,2-diphenylhydrazine and 5 equiv. KN(SiMe3)2. Complex 3 was isolated as dark brown needles in 65% yield following crystallization from a concen- trated pyridine solution of the reaction mixture layered with hexanes. Scheme 1 Synthesis of the lithium manganese diphenylhydrazido complex.40 X-ray crystal structures revealed that 1 and 2 both formed formally Ce(IV) complexes by charge balance, with four dia- nionic diphenylhydrazido ligands and four alkali metal cations would similarly form cerium complexes with secondary struc- per cerium cation in the formula unit (Fig. 1). Within the tures governed by alkali metal cations. structures, the alkali metal cations bridged neighbouring Dark purple Li4(py)4[Ce(PhNNPh)4](1) and Na4(py)8- 1,2-diphenylhydrazido units. Surprisingly, 3 formed an [Ce(PhNNPh)4](2) were synthesized by layered reactions of Ce extended coordination polymer in which the potassium ions [N(SiMe3)2]3 with 4 equiv. 1,2-diphenylhydrazine and 4 equiv. interacted both intramolecularly through bridging neighbour- ¼ MN(SiMe3)2,M Li or Na, in a mixture of Et2O and pyridine. The ing hydrazido ligands and intermolecularly through K–arene yields for 1 and 2 were 75% and 63% respectively (Scheme 2). interactions within the ligands at K(4) and K(5) (Fig. 1). The most notable difference in the structure of 3, however, was the presence of an additional K+ cation per formula unit, indicating that 3 was a formally Ce(III) complex. The N–N bond lengths in complexes 1, 2, and 3 were consistent with single bonds ranging from 1.451(2)–1.466(3) A˚ 39,42–46 – Creative Commons Attribution 3.0 Unported Licence. (Table 1). The Ce N distances for 1 ranged from 2.4199(13)–2.4408(14) A˚ while those for 2 were slightly shorter at 2.373(2)–2.398(2) (Table 1). The shortened Ce–N distances for 2 compared to 1 were consistent with the stronger Lewis acidity of Li+ cations in 1 versus Na+ cations in 2. The Li+ cations reduced the relative charge density at the nitrogen atoms for binding with the cerium cation, compared to the Na+ cation in 2. This effect was reversed in 3, however, with Ce–N bonds ranging from 2.449(3)–2.636(4), in support of bonding to the larger This article is licensed under a Scheme 2 Syntheses of complexes 1, 2, and 3. Open Access Article. Published on 11 August 2015. Downloaded 9/27/2021 7:22:12 AM. Fig. 1 30% probability thermal ellipsoid plots of Li4(py)4[Ce(PhNNPh)4](1) (left), Na4(py)8[Ce(PhNNPh)4](2) (middle), and K5(py)7[Ce(PhNNPh)4](3) (right) with the phenyl and pyridine rings shown in wire frame. Hydrogen atoms were omitted for clarity. Selected bond distances for 1 (A):˚ Ce(1)– N(1) 2.4408(13), Ce(1)–N(2) 2.4199(13), N(1)–N(2) 1.451(2), Li(2)–N(1) 2.018(3), Li(2)–N(10) 2.018(3), Li(1)–N(2) 1.995(3). Selected bond distances for 2 (A):˚ Ce(1)–N(1) 2.390(3), Ce(1)–N(2) 2.373(2), N(1)–N(2) 1.462(3), Na(1)–N(1) 2.853(3), Na(1)–N(2) 2.630(3), Na(1)–N(3) 2.535(3). Selected bond distances for 3 (A):˚ Ce(1)–N(1) 2.564(3), Ce(1)–N(2) 2.480(4), N(1)–N(2) 1.465(5), K(2)–N(1) 3.044(4), K(2)–N(2) 2.877(4). 6926 | Chem. Sci.,2015,6, 6925–6934 This journal is © The Royal Society of Chemistry 2015 View Article Online Edge Article Chemical Science Table 1 Unique Ce(1)–N and N–N bonds and the tabulation of s4 parameters for complexes 1, 2, and 3 measured by X-ray crystallography or DFT calculations ˚ ˚ a ˚ ˚ a b a,b Complex Ce(1)–N(x) (exp, A) Ce(1)–N(x) (calc, A) N–N (exp, A) N–N (calc, A) s4 (exp) s4 (calc) 1 2.4408(14) 2.464 1.451(2) 1.441 0.110 0.000 2.4199(13) 2 2.390(3) 2.439 1.462(3) 1.441 0.663 0.498 2.373(2) 2.443 1.461(3) 2.380(3) 1.457(3) 2.381(2) 1.466(3) 2.398(2) 2.374(2) 2.397(2) 2.394(2) 3 2.564(3) 2.582 1.465(5) 1.448 0.773 0.709 2.480(4) 2.488 1.449(5) 2.415(4) 1.459(5) 2.636(4) 1.456(5) 2.499(3) 2.482(4) 2.449(3) 2.494(3) a Pyridine was replaced with OMe2 in the calculated structures, resulting in the following calculated complexes: Li4(OMe2)4[Ce(PhNNPh)4], À b Na4(OMe2)4[Ce(PhNNPh)4], and K4(OMe2)4[Ce(PhNNPh)4] .